fix: crash in lazy mode

In lazy mode, res.locals.webpackStats is undefined

lib/router.js

@@ -27,15 +27,7 @@ function testPathToChunkUrl(testPath) {
  */
 function middleware(routeBuilder) {
     return function middleware(req, res, next) {
-        const commonAssetsObj = _.omitBy(
-            res.locals.webpackStats.toJson().assetsByChunkName,
-            value => /\.bundle.js$/.test(value)
-        );
-        const commonAssets = Object.keys(commonAssetsObj).reduce((result, item) => {
-            result.push(assetsRoot() + commonAssetsObj[item]);
-            return result;
-        }, []);
-
+        const commonAssets = readCommonAssets(res.locals.webpackStats);
         log(`request to ${req.path}`);
         if (routeBuilder.isUrlRegistered(req.path)) {
             log('this is a test page url');
@@ -48,6 +40,19 @@ function middleware(routeBuilder) {
     };
 }
 
+function readCommonAssets(webpackStats) {
+    if (!webpackStats) {
+        return [];
+    }
+    const commonAssetsObj = _.omitBy(
+        webpackStats.toJson().assetsByChunkName,
+        value => /\.bundle.js$/.test(value)
+    );
+    return Object.keys(commonAssetsObj).reduce((result, item) => {
+        result.push(assetsRoot() + commonAssetsObj[item]);
+        return result;
+    }, []);
+}
 
 exports.testPathToChunkName = testPathToChunkName;
 exports.testPathToChunkUrl = testPathToChunkUrl;